Pahlavan believes the tilt to be the result of gravitational pulls exerted on large lumps of debris left over from the formation of the inner planets billions of years ago.

According to the Giant Impact Theory, the leading explanation for the origin of the moon, a Mars-size rock called Theia struck the Earth about 4.5 billion years ago. The resulting debris came together and became what is now the moon.

The Moon's inclination is measured with respect to the ecliptic, the Earth’s orbital plane. The Earth’s axis is tilted at 23.4° with respect to this plane. While it is expected that the Moon’s tilt be close to that of the Earth, it actually has a 5° deviation from the orbital plane. This discrepancy has been called “the lunar inclination problem.”

Pahlavan and his team have proposed that the Moon's orbit was influenced not only by the initial collision, but also by further close encounters - the influence and intensity of which is a function of how close to the moon they occured. Numerous computer simulations were used in this study to arrive at an estimate of the aggregate effects of such close encounters on the lunar orbit.

"This discovery places the Moon-forming event into the context of Earth formation," said Pahlavan, in an interview with IFLScience. "About one percent accretion onto the Earth is required to explain the chemistry of the Earth's mantle. We have shown that the same amount of accretion can reproduce the excitation of the lunar orbit. Hence, this discovery presents a coherent picture of the final stage of accretion of the Earth following the Moon-forming event."